How to Calculate Crowd Size at Big Events, Like the Inauguration

how to calculate the size of a crowd, whether they are gathering on the National Mall in Washington, D.C., or on a college campus:

The method goes back to the late 1960s and a University of California at Berkeley journalism professor named Herbert Jacobs, whose office was in a tower that overlooked the plaza where students frequently gathered to protest the Vietnam War. The plaza was marked with regular grid lines, which allowed Jacobs to see how many grid squares were filled with students and how many students on average packed into each grid.

After gathering data on numerous demonstrations, Jacobs came up with some rules of thumb that still are used today by those serious about crowd estimation. A loose crowd, one where each person is an arm’s length from the body of his or her nearest neighbors, needs 10 square feet per person. A more tightly packed crowd fills 4.5 square feet per person. A truly scary mob of mosh-pit density would get about 2.5 square feet per person.

The trick, then, is to accurately measure the square feet in the total area occupied by the crowd and divide it by the appropriate figure, depending on assessment of crowd density. Thanks to aerial photos or mapping applications like Google Earth, even outdoor areas can be readily measured these days.